Circuit and method for equalizing erase rate of non-volatile memory cells

ABSTRACT

A pre-erase/incremented erase mechanism is employed to reduce excessive tunnel oxide fields in flash memory cells. A variable conditioning signal removes charge from “fast” bits in the array, so that they are configured to have threshold voltages closer to an ideal initial state in preparation for an erase cycle. In this manner, the voltage thresholds are tightened and equalized, so that over-erasure problems associated with Fowler-Nordheim tunneling erase operations are substantially reduced, and endurance cycles for the array are maximized. The invention can be used in a device in the field, or as part of a design process for a flash memory cell to evaluate device performance.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of application Ser.No. 09/344,318 filed Jun. 24, 1999.

FIELD OF THE INVENTION

The invention relates generally to a pre-erasing/erasing mechanism forflash EEPROM (Electrically Erasable and Programmable Read Only Memory)devices. The invention is particularly useful for flash arrays thatutilize Negative Gate Channel Erase (NGCE) operations because itimproves erase operations by reducing field intensity variations intunnel oxides caused by processing deviations, increases the number ofendurance cycles, and results in fewer over-erase errors.

BACKGROUND OF THE INVENTION

The endurance cycles of an EEPROM, especially the number of program anderase cycles, is the primary determinant of the life span of suchdevice. Endurance cycles refer to the number of times which data can bereliably erased, reprogrammed, and read back without errors.Accordingly, the number of endurance cycles dictates in large part theusable life of an EEPROM device. Consequently, one significant goal ofprior art efforts is the maximization of such endurance cycles throughthe use of improved cell architectures which minimize cycle stress, aswell as intelligent erase, program methods designed to be less stressfulon the cell architecture.

Fowler-Nordheim (FN) tunneling is one of the most well-known,well-understood and prevalent techniques used in the art for erasingflash memory cells. A significant problem arises from the fact thatcertain cells (bits) in the memory array tend to be “fast” and others“slow” during an erase operation. In other words, because such cellstend to over-accumulate, or under-accumulate charge on their floatinggates, or because charge becomes trapped in various locations, thesecells tend to have threshold voltages that deviate significantly from atarget threshold voltage. For this reason, when a “fast” cell is erased,it is much more likely to become over-erased. When a cell is over-erasedduring an erase operation, this causes additional administrativeoverhead, because it must be corrected if at all possible. This isundesirable, of course, because it slows down the operation of thedevice from being re-programmed with new data.

The existence of excess charge on the “fast” bits caused them to beover-erased for a couple of reasons. First, the FN erase mechanismdepends strongly on the electrical field across the thin tunnel oxidelayer in the flash cell; with more charge, the field is proportionatelyhigher. This causes more charge to move off the floating gate during theerase operation. In other words, the fast bits have electrical fieldintensities that deviate significantly from desired target fieldstrengths. Second, at the beginning of the FN erase, the floating gatesare fully charged (i.e., in a programmed state) and the electrical fieldis a maximum. This means that the rate of erasure (the electricaldischarge rate of the floating gate) is also highest at the onset of theFN erase operation. Thus, if fast bits are not corrected, they tend tobecome over-erased, and this leads to concomitant problems of excessleakage current and/or data errors. In the long run, these leaky bitscause failures, reduce endurance cycles, etc. As mentioned above, theproblem is especially acute in (but not limited to) FN tunneling eraseoperations used with NGCE configurations. This is because the electricalfield becomes extremely strong between the negative gate and thesubstrate well.

To date, there are very few practical solutions for dealing with thisphenomenon, and no easy way known to applicants for compensating forelectrical field intensity variations that occur in flash memory cellarrays. These field variations arise naturally both from waferprocessing operations, which, by their nature, result in differences incell structures, tunnel oxide characteristics (thickness anduniformity), etc., as well as from cycling of the device in normaloperation. As the number of cells increase in EEPROM devices, andintegration density increases, and cycling increases, the fieldvariations also correspondingly increase because of the nature of normaldistributions.

A related situation is addressed by U.S. Pat. No. 5,901,089,incorporated by reference herein. In this reference, the individuallogic levels of a multi-bit cell are kept stable by ensuring that thethreshold voltages of such cell are maintained within predefinedthreshold ranges. This is accomplished using what it refers to as “mini”erase/program operations, where a pulse is applied so that only enoughcharge is added or removed from a cell sufficient to keep it within asafe operating range for that state. While this approach is beneficialfor improving sensing (read) operations, this technique does not appearto be very practical as a pre-erase conditioning operation. This isbecause it requires a significant amount of administrative overhead toperform this type of procedure on a cell by cell basis. Moreover, thereference is primarily directed ensuring that Vt overshoot is reducedfor each programmed cell, and does not really address the issue of howto ensure that the behavior of cells collectively is relatively uniformand predictable during a subsequent operation in which they are to betransitioned from one logical level to another. In other words, there isno attempt made to treat one segment of the cell population (the “fast”cells) differently than another as may be necessary to achieve betteroperational results. Nor does this reference teach or suggest using aseries of lesser strength erase signals to completely erase a programmedcell.

For these reasons, an effective method for dealing with so called fastbits is not known in the art, but is extremely desirable. Optimally, theerased set of cells has a relatively uniform distribution centeredaround a target erase value. To get this uniform distribution, however,it is necessary: (1) to place the cells in a similar state (initialfloating gate voltage/electrical field); and (2) for them to havesimilar electrical discharge characteristics. The first step can beaccomplished by a conventional programming step, which, in theory, isintended to add sufficient charge to put all the cells into an initialtarget programmed voltage threshold state V_(p). To date, nonetheless,applicants are unaware of specific and practical mechanisms forachieving the second step of controlling individual cell electricaldischarge characteristics. To accomplish this goal, it is necessary tocontrol the initial electric field at the tunneling oxide. If thiselectric field is not reduced at the onset of FN erasure for the fastbit segment of the array, the fast bits will be erased faster thanaverage bits in the memory array. By the time the average bits areerased, the fast bits are already in a state of over-erasure.Correspondingly, if the electric field can be reduced at the beginningof the erasure for a particular bit, this will slow down the rate oferasure for the bit in question.

Another object of the present invention is to identify fast bits in aflash memory array and provide them with conditioning signals whichmodify erase behavior of such fast bits but not erase characteristics ofaverage or slow bits in such array;

A related object of the present invention is to provide an erasemechanism that uses a stepped or graduated reduction of charge for cellsin a flash memory, so that erase cycles and resulting erased voltagedistributions are more accurately controlled;

Yet a further related object is to reduce cell leakage current byeliminating the occurrences of over-erased fast memory bits in a flashmemory array;

Still another object of the present invention is to provide a memorycell array capable of longer life span, by increasing the number ofendurance cycles;

An additional object of the present invention is to reduce thepossibility of malfunctions and errors in flash memory cells caused byexcessive numbers of leaky data bits.

Yet still another objective of the present invention is to provide acomplete integrated circuit that implements the aforementioned methods.

Another objective of the present invention is to provide an improvederase circuit for accomplishing both the regular full strength NegativeGate Channel Erase and pre-erase operations described above, using thesame charge pump.

A further object of the present invention is to provide a programmedlogic controller for achieving step-by-step time sequencing of varyingintensity conditioning signals to effectively reduce the electricalfield across the tunneling oxide at the initial phase of erasure inorder to control/equalize the discharge rate of fast bits in Flashmemory array.

These objectives and other significant advantages are provided by thenovel methods and circuits disclosed herein. A method of slowing downerase speeds of “fast” discharge flash cells in a memory array—where thefast discharge flash cells are generally characterized by erase speedssubstantially faster than target erase speeds for flash cells in thearray—generally includes the following steps: (a) generating aconditioning signal to remove a quantity of charge from the flash cells,which quantity of charge is related to an erase speed of the flash cell,but is insufficient to place such flash cells into an erased state; and(b) applying such conditioning signal to such flash cells while thecells are in a non-erased state.

SUMMARY OF THE INVENTION

An object of the present invention, therefore, is to eliminate theproblems generally inherent in the aforementioned FN tunneling eraseoperations when used with NGCE type EEPROM devices;

Another object of the present invention is to provide an improvedcircuit and method for erasing flash memory cells so that thresholdvoltage distributions of such cells are tightened both before and afteran erase operation;

A further object of the present invention is to provide a circuit andmethod for pre-erasing flash memory cells with a conditioning signal toadjust and equalize their threshold voltage distributions prior to anerase operation;

A related object of the present invention is to provide a circuit andmethod for generating such conditioning signal, so that thresholdvoltage distributions can be finely controlled by a series ofconditioning signals having predetermined magnitudes, durations, andsimilar signal characteristics;

Another object of the present invention is to provide that suchconditioning signals can be custom tailored based on the particularprocessing implementation for such memory cell array, or, alternatively,that such signals can be determined in the field during normal operationof a flash EEPROM device;

Yet another object of the present invention is to control and equalizeelectrical field intensities across a tunneling oxide for flash memorycells in a memory array, and to compensate for normal manufacturingvariations causing such field intensity deviations;

A related object of the present invention, is to reduce a discharge rateof fast bits in a flash memory cell to make such fast bits discharge ata rate comparable to a an average flash memory cell having a desireddischarge rate;

Another related object of the present invention is to reduce theprobability of a fast bit from becoming over-erased, by conditioningsuch bit to have an electrical field comparable to that of an averageflash memory cell prior to an erase operation, so that such fast bitbehaves essentially like an average memory cell during such eraseoperation, and is therefore not over-erased by a full strength erasepulse; and The conditioning signal is preferably applied on a globalbasis to the control gate while the source and drain regions are keptfloating. In this fashion, the floating gate charge value is adjusted bythe conditioning signal by a charge amount proportional to the originalelectrical field intensity. In other words, the faster cells willexperience the greatest change in charge reduction. From a physicalperspective, therefore, the present invention operates to substantiallyequalize electrical field intensities across tunnel oxides associatedwith each of the floating gates of the flash memory cells.

After the conditional signals treat such array, the erase speed of suchfast discharge flash cells is significantly reduced. Since the othernon-fast cells are left relatively unaffected, the entire distributionof erase speeds (and threshold voltages) in the cell array is equalized,so that the array can be treated with greater reliability andpredictability in subsequent array operations (such as an eraseoperation).

Prior to performing the above conditioning, of course, a conventionalprogramming operation can take place, followed by an additionalthreshold voltage measuring step. The latter measures the electricalcharacteristics of the flash cells to determine an expected erasedischarge speed for such flash cells, and to determine which ones, ifany, may need pre-erase conditioning.

During the conditioning cycle, the above steps are repeated as necessaryto remove further charge from the flash cells. For each iteration, theproperties of the conditioning signal, including magnitude and/orduration, can be varied to remove more or less charge from the flashcells. Usually, the cycle is set up so that each successive conditioningsignal removes a greater quantity of charge.

After the array is pre-conditioned, a normal, full strength eraseoperation can be performed. Because the cell discharge characteristicsare modified immediately in advance of the erase operation, the resultis that there are fewer fast bits, and those that do exist have beenslowed down to the point where it is less likely they will becomeover-erased.

In another embodiment of the invention, instead of a full strength erasecycle, the aforementioned conditioning cycle is instead repeated asnecessary to fully erase the cells. This approach provides finer, moreaccurate control of the erase operation, and helps to ensure that, asthe cell array characteristics change with time, the erase is performedin a manner that is tailored to such characteristics.

Thereafter, an over-erase detection operation takes place, to verifythat no cells have been over-erased from said erase operation. In theevent any such cells are detected, a two-stage convergence operation ispreferably performed. This includes generally two independent sequentialoperations, including: (a) a global operation performed on the entirearray; and (b) a localized operation performed only on those cells stillover-erased after the global operation is completed.

In another variation of the invention, it is possible, for example, topractice the present methods in the context of a pre-programconditioning operation, so that the cell charge acquisition (instead ofdischarge) characteristics are modified instead. The only importantconsideration is that the flash cell floating gate charge values aremade to substantially converge to an initial target charge value so thatall of the flash cells can then be erased/programmed at an approximatelyuniform rate by a subsequent operation.

Another beneficial aspect of the present invention lies in the fact thatcharacteristics of the conditioning signal can be adjusted during amanufacturing operation, so that it will be specifically tailored forthe manufacturing deviations present in an associated memory array.

A novel method of erasing flash memory cells in a flash memory cellarray, therefore, utilizes the aforementioned pre-erase operation inconjunction with a traditional erase operation. Again, unlike aconventional erase operation, however, the present invention can “fix”potentially problem cells individually for maximum control/flexibilityof an erase operation. Thus, non-uniformities in electrical fieldintensities across tunnel oxides for flash cell in such array, caused byunpredictable but inevitable manufacturing process variations, can besubstantially reduced. This results in increased product endurance, lesserase time, better yields, etc.

A flash memory integrated circuit can be constructed using conventionalwafer processing steps to include the teachings of the presentinvention. In such circuit, an array of flash memory cells is provided,each of such cells having a conduction threshold voltage, a controlgate, a floating gate, a source and a drain. A logic controller circuitis coupled to the array for adjusting the threshold voltages of thecells prior to an erase operation, and is configured such that it cangenerate control signals in accordance with the methods described, andso that appropriate signal generating circuits can apply theaforementioned conditioning signals in the manner described above aswell.

A preferred embodiment of a pre-conditioning control circuit of thepresent invention effectuates the above processes by the operation of: apre-erase signal generator for generating a flash cell pre-erase signalto be applied to the flash cells; and a pre-erase signal magnitudecontrol circuit for generating a plurality of control signals, whichcontrol signals determine a magnitude for the pre-erase signal; apre-erase voltage supply circuit for generating a number of necessarysupply voltages, including a negative voltage source in the case of apre-erase signal; a pre-erase voltage supply switching circuit forgenerating a pre-erase voltage supply signal that is based on thenegative voltage source; and a pre-erase voltage supply switchingcontrol circuit, which is configured to control gating of the pre-erasevoltage supply switching circuit with a stepping signal. Thepre-conditioning control circuit thus generates the pre-erase voltagesupply signal as a function of the plurality of control signals and in atime sequence determined by the pre-erase signal magnitude controlcircuit This flash cell pre-erase signal is configured, as notedearlier, to remove a quantity of charge from the flash cells inpreparation for an erase operation, which quantity of charge is relatedto an initial erase speed of the flash cell and further beinginsufficient to place such flash cells into a fully erased state.

Another useful aspect of the inventive pre-conditioning control circuitincludes the fact that the pre-erase signal generator includes aplurality of FETs connected in parallel, and that are activated by theplurality of control signals according to a controllable sequence sothat the desired magnitude and duration of the pre-erase signal is wellcontrolled.

As noted earlier, the control signals are varied during the timesequence so that the magnitude of said pre-erase signal is varied duringsaid defined time sequence. In the typical case, it is increased fromstep to step to affect the cells more—i.e., to remove more and morecharge during each step. Again in a preferred embodiment, a pre-erasesignal is first pre-charged to a target reference voltage level beforethe plurality of control signals are applied, to increase speed andreliability.

To control the pre-erase voltage supply switching control circuit, aplurality of pulse generator circuits, one for each of the plurality ofcontrol signals, generate a pulsed version of such control signals thatis applied and used to generate the aforementioned stepping signal.

The aforementioned preconditioning circuit, therefore, operates as anequalizing circuit that equalizes the erase rates of the flash cells,and thus makes it more likely that they will not be over-erased,requiring additional system overhead, loss of data from faulty bits,etc.

As alluded to earlier, the operational characteristics of the pulsegenerator circuits can be adjusted during a manufacturing operation, totailor such operational characteristics to the pre-erase signalrequirements of cells in a flash memory array associated with thecontrol circuit.

Thus, a system for erasing flash memory cells in the present inventionincludes programmed logic within a flash controller circuit (usually astate machine), so that erase control logic includes two logicalcomponents, including: (1) a pre-erase controller circuit controlling aflash cell pre-erase operation, managing the conditioning signalmagnitude, timing and sequencing, as well as (2) a conventional erasecontroller circuit for controlling a normal strength flash cell erasesignal.

The present invention, therefore, is extremely useful in thoseenvironments that rely on FN tunneling, and helps to eliminate potentialproblems caused by normal process variations in tunnel oxide layers,floating gates, cell topographies, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified flow chart showing steps used in a conventionalprior art non-volatile Flash memory erase operation.

FIG. 2 is a flow chart showing the steps practiced by a flash memorylogic controller circuit of the present invention, including a pre-eraseoperation performed prior to a full erase.

FIG. 2B is a cross sectional view of a non-volatile Flash memory cellstructure used in the present invention, which cells are situated in aP-WELL enclosed by a deep N-WELL.

FIG. 3 depicts in block diagram format the voltage generators and signalgenerators used for performing the pre-erase and modified eraseoperation of the present invention.

FIGS. 4A-4C are more detailed circuit schematics of aspects of a flashmemory conditioning signal generating circuit relevant to the presentinvention.

FIG. 4D is a table showing the sequence and magnitude of variousconditioning signals generated by the present controller circuit.

FIG. 5 illustrates a first embodiment for generating a control pulse inresponse to the logic controller circuit using a one-shot delay circuitschematics comprised of a MOSFET device and capacitor elements.

FIG. 6 illustrates a second embodiment for generating a control pulsefor the logic controller using a one-shot delay circuit comprised ofresistor and capacitor elements.

FIG. 7A is a graphical illustration of the effect of a prior art eraseoperation on the threshold voltage of a slow flash cell and a fast flashcell in a memory cell array.

FIG. 7B is a graphical illustration of the effect of the pre-eraseoperation of the present invention on a threshold voltage for fast flashcells in a memory array.

FIG. 7C is a graphical depiction of the effect of the pre-eraseoperation of the present invention on floating gate electrical chargedischarge characteristics for flash cells in a memory array.

FIG. 8 depicts a preferred embodiment of a well bias voltage circuitused in the present invention;

FIG. 8A illustrates a timing diagram of a voltage level as it isgenerated by the well bias voltage circuit and applied to various wellareas of a flash memory array.

DETAILED DESCRIPTION OF THE INVENTION

A detailed explanation of the preferred embodiments is now provided,examples of which are illustrated in the drawings and discussed herein.

In a conventional non-volatile Flash EEPROM array, data in the memoryarray may be erased electrically all together as a block while new datacan be programmed after on a byte-by-byte basis. Although the erasure isnormally effected on all memory cells in the array simultaneously, notevery cell in the same array will be erased equally fast, or to the sameerased voltage state as mentioned above. Some bits will be relativelyfast, and some bits will be relatively slow. This is due primarily tonormal variations in the processing technology as well as thetopological deviations associated with the control circuits, which tendto affect the tunneling oxide characteristics, the floating gatecharacteristics, etc. In addition, normal “aging” of a cell fromrepeated cycling will also cause it have varying and unpredictablebehavior due to charge trapping, structural deterioration, and the like.These factors are unavoidable in conventional memory arrays, and arebecoming increasingly more difficult to cope with as such memory arraysincrease in density, size, etc. Thus, owing to the reasons cited above,controlling the erase operation in a Flash memory array to achieveuniform memory cell characteristics after electrical erase is becomingsubstantially more a critical than the program operation.

A conventional method of erasing a Flash memory array is illustrated inFIG. 1. As is known, the memory array is first pre-programmed at step 20to set all cells to a known data value before the erase operation. Afterconfirming at step 30 that all cells have passed the programverification, the procedure then proceeds to erase the cells at step 40using any conventional method, such as a standard erase pulse ofsuitable magnitude and duration applied repeatedly to the entire array.The execution of an erase operation involves typical logic controlcircuits such as a counter, timer, pulse generator logic controller andso forth, which are well-known in the art. An example of these circuitscan be seen in our pending application Ser. No. 08/988,872 which isincorporated herein by reference. At step 50, a check is made for overerased bits; these bits are undesirable for a number of well-knownreasons, including the fact that they tend to cause current leakage inthe memory array. If over erased bits are detected by the controlcircuits, step 60 then tries to perform a correction routine to reducethe leakage current. One exemplary method for performing such routine isdescribed in detail in the aforementioned pending application. Thiscorrection procedure is repeated until all bits can pass the leakagecurrent test, or until there is a determination made that the part hasfailed due to uncorrectable current leakage in the Flash memory array.

As soon as step 50 and step 60 are finished, step 70 executes an eraseverification routine to ensure all the memory cells in the Flash memoryarray are completely erased. If any cell in the memory array is not yeterased completely, the control circuits starts the entire procedure overagain as illustrated in the flow chart of FIG. 1. If for any reason,however, the memory array can not pass any step as shown in the flowchart, the Flash memory array is considered defective, and thus theendurance cycles are compromised to a much lower limit. It is plain alsoto see that the existence of over-erased cells significantly slows downthe operation of the device, since routine 60 must be executed to see ifsuch cells can be corrected. Furthermore, the correction routine 60 anderase operations 40 impose additional stresses on the cells by way ofadditional conditioning signals/pulses, and this also undesirablyshortens the life span of such cells.

Clearly, therefore, it is extremely beneficial to reduce the need andinvocation of such routines; applicant believes that the presentinventions accomplish this goal by taking intelligent precautions toreduce the probability that any cells will become over-erased in thefirst place. This is done primarily by a strategy that focuses on thosecells that are identified as susceptible or pre-disposed for erraticdischarge behavior, and in particular, so-called “fast” bits which aremost likely to cause such problems.

FIG. 2 illustrates an embodiment of a preferred erase method of thepresent invention, which, in contrast, to the prior art includes apre-erase procedure as well. This new erase and pre-erase method is nowexplained briefly. Generally speaking, the primary task of the pre-eraseprocedure is to correct (as best as practically possible) the fast bitsin the non-volatile Flash memory array, by reducing an electrical fieldassociated with a tunneling oxide for such bit. From a simplifiedperspective, and with all other things being equal, as the electricalfield across the tunneling oxide becomes greater, so does the rate ofcharge accumulation or discharge for the floating gate of the cell.Accordingly, if in advance of an erase operation this tunnel oxide fieldcan be reduced for those cells having relatively high initial startingfield values, such cells can be effectively slowed down and made tobehave more like a target cell during the actual erase operation.Furthermore, if this electrical field treatment can be controlled withreasonable precision, the pre-erase procedure can be confined andlimited to affect only those cells that are the most likely to cause aproblem during the erase operation. Thus the present method has littleor no adverse effect on any cells having normal or slow behavior duringan erase operation.

The effect of the pre-erase treatment can be summarized as follows:

the electrical field across the tunneling oxide is reduced by an amountproportional to an initial value for such field; in other words, thosecells having the greatest positive deviation from a target field value(i.e., those having the greatest potential for over-erasure) areaffected the most;

this means that the erase speeds of these fast discharge flash cells arereduced to be more like those of target flash cells, so that for a givenerase pulse magnitude and duration, such fast cells do not lose as muchcharge as they would before without pre-erase treatment;

the corollary of this is that the overall erase speed of cells in thememory array is “equalized” in the sense that the fast bits are made toconform to look like the rest of the cell population;

because the overall cell population discharge rate is slowed down, andthe fast cells are not as fast, they do not lose so much charge thatthey become over-erased;

the reduction in probability of a cell becoming over-erased results infewer over-erased cells in fact, which in turn means that feweradditional corrective routines and erase routines are executed, andwhich further means increased endurance and life-span for the devicebecause of the reduction in cycle stressing;

with the pre-erase cell population more uniformly distributed, the eraseoperation has more predictable and consistent effect, and the resultingerased population also has better threshold voltage distributions;

with a more predictable erased threshold voltage distribution for thecells in the memory array, later operations, including programming, arealso more reliable;

undesirable processing variations in the tunneling oxide, cell arraytopographies, etc. that would otherwise cause premature failures can becompensated for (to a large extent) in a manner that is inexpensive,flexible, and easy to implement

The routine of FIG. 2 is preferably executed as a control routine by alogic controller coupled to the flash memory array (see FIG. 2B). Asseen at step 100, this routine starts by verifying that a command ispresent for initiating an erase operation. If the commands are correct,then an internal state machine of the logic controller sets a variableERGO=1 valid. The logic controller circuit and state machine can beconstructed in any number of known ways, and the specifics of the sameare not material to the present invention. This is true, as well, withother hardware aspects of the present invention with the exception ofthe voltage generators (FIG. 3), pulse generators and control logic(FIGS. 4-6) for which preferred circuit implementations are specificallyprovided for herein.

Referring again to FIG. 2, after the initial erase command verificationstep, a conventional pre-programming before erasure step takes place at110; the intent of this step is to move the threshold voltages of thecells to a known state prior to the erase operation, and to reduceover-erase problems. Thus, at this stage, the cells are all in aprogrammed state, which can also be considered a non-erased state; i.e.,the threshold voltage V_(t)=V_(p), where V_(p) is a target programmedstate value. Ideally the cells will have the same initial floating gatecharge, and the same threshold voltage value at this point, but, for thereasons mentioned above, this is not the case in reality. If thepre-program fails, the routine routes to step 180 for hang-up whichmeans the memory array is defective, which may be caused by a number ofreasons.

At step 120, if all the programming procedures are completed, instead ofbeginning a conventional erase operation, the pre-erase procedure of thepresent invention is executed. During this pre-erase procedure, a seriesof conditioning signals (pulses) are applied to the cells in the memoryarray. These signals are configured to remove a quantity of charge fromthe floating gates of the cells, which charge quantity is enough toreduce the tunnel oxide field, but not enough to put the cell into anerased state. As seen in FIG. 2B, the pre-erase conditioning signals areapplied to a control gate VCG while the source and drain are leftfloating. A positive potential is asserted at both the p-well and n-wellregions while the p-substrate is grounded.

In practical terms, this step can be considered a “partial” erase, sincesome charge is removed from the floating gates of the cells, but notenough to constitute a fill strength erase. Unlike a full strengtherase, the conditioning signals in this case are only applied for shortperiods of time, for a limited number of cycles, and can be smaller inmagnitude as well. Broadly speaking, the preferred approach begins witha conditioning signal that causes a relatively low electricalcounter-acting field to be generated across the tunneling oxide toremove charge from the floating gates, and this counter-acting field isgradually increased in subsequent conditioning signals to some maximumvalue. Notably, when these conditioning signals are applied to thecells, the cells with the highest initial fields will also experience agreater loss of charge, which means they will be affected the most. Thisbeneficial side effect of the present invention means that relativelyslow bits, and even normal bits, will be only slightly (if at all)affected by the pre-erase procedure. As mentioned also above, in theend, the fast bits will be modified to have smaller electrical fields,and this in turn results in their behavior being changed to act morelike an ideal, target cell in the array.

Thus, after the pre-erase treatment, the cells will now have a moreuniform, equalized field intensity and correspondingly a more uniformand equalized erase rate. The cells in the array can be monitored, afterthe end of each particular conditioning signal (or some predeterminedsequence) to examine the effects of the treatment to that point. Inother words, the existence, degree, and characteristics of any remainingfast bits can be determined, by examining the threshold voltagedistributions. If necessary, an additional conditioning signal (orsequence) can be applied, followed by a subsequent monitoring step, etc.Thus, a fast, iterative process can be employed by repeating the abovesteps to force the cell population to have a set of threshold valuesbelow a target threshold value, and without concern for placing suchcells accidentally into an erased state, or the possibility ofsubstantially affecting normal cells. The particular sequencing,magnitude, duration, and number of cycles of the conditioning signalswill, of course, vary from array to array depending on thecharacteristics and number of fast bits, but can be optimized asnecessary and/or desired within the logic controller circuit. Aparticular preferred approach used in the present invention is depictedin FIG. 4D, and is discussed in more detail further below.

As soon as the step 120 for pre-erase has completed as illustrated inFIG. 2, erase verification at step 130 commences to check if all thememory cells have been completely erased. While no cells are intended tobe fully erased prior to this time, this step is common practice, andtypical of erase procedures known in the art. If any bit in the memoryarray does not pass erase verification (as would be expected to be trueat this stage), then full strength of erasure at step 140 begins. Thiserase operation uses a conventional full strength erase pulse with amagnitude, duration, and frequency adapted in known fashion for thememory array in question. At the end of this operation, all the memorycells should be in an erased state, and, because of the effect of thepre-erase operations noted above, with reduced numbers of over-erasedcells.

After each erase operation, cell leakage for each column will be checkedas illustrated at step 150 to ensure that no memory cell has beenover-erased in the memory array. If all the cells are erased, with noover-erasures, then the erase procedure terminates.

However, if current leakage in the memory array is detected, then theroutine transitions to a convergence sub-routine comprised of globalconvergence step 170 and column threshold convergence step 160. Apreferred embodiment of this convergence sub-routine is described inapplicant's pending Ser. No. 08/988,872. Briefly, when an over-erasecondition is found for any bit, a FLAG=0 condition exists, so a globalconvergence cycle 170 utilizes a series of “soft” program and soft eraseconditioning pulses to adjust the flash cell threshold voltages in thearray on a global basis. This procedure is executed for a configurable,pre-determined length of time for efficiency and effectiveness based onparameters of the array that can be programmable and/or determinedoptimally in advance by a variety of test and modelling measurements.The soft program and soft erase pulses are also tailored specificallyfor the array in question, and are not of the same type as a fullstrength program and erase pulse respectively. The effect of step 170 isto gently pull and tug the edges of the cell threshold voltages,resulting in a tighter distribution. This is because cells havingrelatively high deviations from the target erase voltage are affectedthe most by these soft program/erase pulses. While the details of theglobal convergence are not material to the present invention, moreinformation on the same can be found in the aforementioned pendingdisclosure.

After the global convergence cycle step 170, cell leakage current ischecked again at step 150. If step 170 has been completed and excessiveleakage current is still detected in the memory array, FLAG is set to 1,and a column threshold convergence cycle 160 is executed to provide eventighter control of cell threshold distribution in the Flash memoryarray. This step is more effective than the global convergence cycle ateliminating single bit over-erase errors, because it is applied on anindividual column basis. More particularly, this cycle uses a series ofadjustable but well targeted soft program pulses to nudge a particularcell out of the over-erased state, and into an acceptable areacorresponding to a threshold value for a target erase state. Theduration of the cycle, as well as the number of soft program pulses, canbe customized for specific applications.

In the applicant's experience, the combination of these two cyclessubstantially reduces the existence of over-erased cells, should anyexist after the erase operation noted above. This combination is optimalbecause the global convergence cycle works very quickly on a rough scaleto eliminates a large portion of leaky cells; the remaining small numberof leaky cells are eliminated on an individual basis by the columnthreshold cycle, which, albeit slower overall, should not slow down theoperation of the device since it should not have many errors to fix.

After step 160 is finished, verification for the completion of erasureat step 130 is repeated to ensure that no memory cells were disturbed bythe convergence cycles at steps 160 and 170. The entire sequence ofsteps beginning at step 130 can be repeated for a pre-determined numberof times, based on a programmable parameter ECOUNT, until the memoryarray passes erase verification, or until such parameter maxes out(ECOUNT=Max), which results in a hang-up at step 180.

In another variation of the above embodiment, the array conditioningprocess is not discontinued prior to an erase operation. Instead theconditioning process is extended in time so that the cells in the arrayare actually finally fully erased, as they would be from a conventionalfull scale erase pulse. In this fashion, the array is erased in anincremented, controlled approach, with more and more charge graduallyremoved from the cells. This is done as follows: during the pre-eraseconditioning process described above, the conditioning signal is stepped(incremented) in magnitude after a predetermined number of cycles (e.g.,8). For the entire pre-erase cycle, the signal magnitude can be variedfrom approximately −6.50 to −8.50 volts, with 0.25 volt increments;however, it is possible that it may be terminated prior to that time, ifthe measured threshold voltage distributions suggests that it is notnecessary to continue (i.e., the fast cells have been slowed downsufficiently). At this point, in lieu of performing a conventional erasecycle, with a large number of iterations of a full scale signal, thepresent embodiment continues the pre-erase cycle past the normalstopping point, and uses this process instead to proceed to fully erasethe array. Thus, instead of breaking up the erase operation into twodistinct phases (i.e., pre-erase and full erase) having different signalcharacteristics (as noted earlier), the incremented erase cycle of thisvariant of the present invention is actually comprised of a single typeof erase operation in which repeated small intensity signals are used togradually reduce the floating gate charge, and, correspondingly, thethreshold voltages of the cells. In this approach, after steppingthrough the various signal levels for each invocation of the process in0.25 volt increments, a final high intensity voltage level is reached(−8.50 volts in a preferred embodiment) for the pre-erase phase. For the“erase” phase, this voltage is kept constant, and there is no longer anystepping of the conditioning/erase signal voltage. This is because thelower intensity signals will no longer be effective to move charge awayfrom the floating gates anyway. Unlike a full erase operation, however,the incremented erase of the present embodiment uses a much smallernumber of pulses (for example, 6-10 applications instead of hundreds fora normal erase) to only adjust the cell thresholds by a much smallerdegree. After each 6-8 pulse iteration, the cell array is checked to seeif it is yet fully erased. If not, another 6-10 pulses are applied, andthe procedure is repeated as necessary. Other variations are of coursepossible, and may be more optimal in some environments. For instance, itmay be preferable to use a scaled number of pulses with each iteration,so that N are used for the first iteration, N/2 for the second, N/3 forthe third, etc., etc. This allows an erase procedure to beself-adjusting to the behavior of the cells in the array, because, overtime, their erase characteristics will change, and it is difficult topredict and fix in advance a constant erase parameter that will satisfyall operating conditions of the array. During this incremented eraseprocess the number of erase pulses, therefore, will vary and bedynamically adjusted in accordance with the requirements of the array.

Notably, the present method can be used with a device in the field, or,alternatively, as a design/test tool aid in connection with monitoringthe quality of a non-volatile cell or array architecture. Examples ofparticular preferred embodiments for generating the pre-erase signals,and associated supporting signals (negative erase supply voltages,control signals, etc.) are illustrated in FIGS. 3 to 6.

In FIG. 3, a block diagram is provided showing the circuit componentsused with a pre-erase/incremented erase circuit 300 to generate thenecessary voltages, clocks, control and pre-erase/incremented erasesignals of the present invention. Oscillating clocks Phi and Phi_(b) aregenerated from block 305 and supplied to negative pumping circuit 310 tointernally generate the required negative voltage V_(NEG) that is usedfor the erase operation (NGCE) used in the present invention. After thenegative voltage is generated on chip, it is distributed by a voltageswitch control circuit 320 to appropriate sections of the Flash memoryarray as signal V_(NGI). This signal, as well as a reference voltageV_(REF) from voltage generator 350 is used by negative voltage levelcontrol (NVLVC) circuit 330 to generate the pre-erase conditioningsignals described above in connection with step 120 (FIG. 2). Generallyspeaking, NVLVC circuit 330 divides the negative voltage V_(NGI) intoseveral smaller negative voltages, and these smaller voltages form thebasis of the conditioning signals applied to the flash cells toeffectively reduce the electrical field across the tunneling oxide.While the negative voltage is divided and distributed to needed sectionsin the Flash memory array, comparator circuit block 340 compares theinternally generated voltage V_(GER) to a pre-determined voltagereference to keep the internally pumped negative voltages at the rightvoltage level.

The aforementioned sections of pre-erase/incremented erase circuit 300can be implemented in a variety of forms suitable for use with thepresent invention. For the present disclosure only portions of NVLVCcircuit 330 need be explained in more descriptive detail, since itprovides part of the novel and unique functionality and features of thepresent invention.

As seen in FIG. 4A, a pre-erase/incremented erase signal generatorcircuit 450 generates Output V_(GER) used for reducing the tunnel oxidefields in response to a combination of various voltage reference andlogic signals. First, a pre-erase/incremented erase voltage switchcircuit 452 provides a negative input voltage V_(NGI) in response tovoltage V_(NEG) from circuit 310, and as gated by signal ESTEPB. Thelatter is active in accordance with signal conditions which can beunderstood with reference to FIG. 4b. Output V_(GER) can be seen as adivided version of negative voltage input V_(NGI); in operation, acapacitor divider formed by capacitor 454, capacitor 460A, and othercapacitor stages that are switched in (460B, 460C, 460D and 460E)depending on the state of certain logic variables ERLVC<3:0> which inturn activate associated FET switches (462B and 464B, 462C and 464C,462D and 464D, and 462E and 464E respectively). For example, if apre-erase conditioning pulse V_(NGI) of approximate magnitude −6.25volts is desired, only ERLVC <0> is activated; all other pre-erasemagnitude control signals (ERLVC <1>, ERLVC <2>, ERLVC <3>) are low.These signals are generated in conventional fashion by apre-erase/incremented erase magnitude control circuit, which isimplemented as part of the logic controller circuit described abovewithin the flash memory integrated circuit. Thus, with this combination,a negative voltage _(VNGI) of approximately −6.25 volts is provided topre-erase/incremented signal generator circuit 450, which voltage isthen divided by the serial combination of capacitor 454 taken with theparallel combination of capacitor 460A and 460B. The latter is switchedin by virtue of the fact that FET 462B is turned on by the state ofERLVC <0>, and the fact that FET 464B is switched off (by a signalERLVCB <0> having opposite logic to ERLVC <0>). Furthermore, no othercapacitor (460C, 460D or 460E) is switched in at this point.Accordingly, a capacitive divider is formed by 454, 460A and 460Bbetween V_(NGI) and ground, with V_(GER) taken at a potential betweencapacitor 454 and capacitors 460A and 460B corresponding toapproximately −6.25 volts. The other possible combinations ofpre-erase/incremented erase magnitude control signals ERLVC<3:0> neededto generate larger magnitude pre-erase/incremented erase signals aregiven in the table of FIG. 4d. It is apparent, of course, thatadditional levels could be used if desired. While thepre-erase/incremented erase signals are being applied there is nocontribution from V_(REF), because FET 456 is not activated by signalESTEP. Instead, V_(REF) is used as a pre-charger for the capacitordivider, and is controlled by ESTEP generated by pre-erase/incrementederase voltage switch control circuit 406.

The origin of signals ESTEP and ESTEPB are shown in FIGS. 4b and 4 c.First, as seen in FIG. 4c, a series of pulse generators 401, 402, 403,404 and 405 are provided. Inputs to these pulse generators are providedin the form of pre-erase/incremented erase magnitude control signalsERLVC<3:0>, and sequence control signal ER, which is also generated bythe logic controller circuit to control the sequencing (time spacing)between consecutive pre-erase/incremented erase conditioning for thepre-erase/incremented erase state machine operations. These pulsegenerators govern the stages of the pre-erase/incremented erase signalV_(NGI) and can be implemented and configured as described further belowwith reference to FIGS. 5 and 6.

Circuit 406 in FIG. 4b, therefore, can be considered as apre-erase/incremented erase voltage switch control circuit for thepre-erase/incremented erase signal generating circuit 450. In thebeginning, V_(GER) is pre-charged to a reference voltage, which is afunction of the ERLVC signals selected, and the effect of ESTEP inactivating FET 456. This signal voltage V_(GER) later on is coupled by acombination of various capacitors between V_(NGI) and ground. Thisone-shot pre-charge of V_(GER) is also supported by the operation ofcircuit 406.

As is apparent, the amount of charge removed from the floating gates ofthe memory cells is a function of the signal duration, and, theirrelative electrical field intensity across the tunnel oxide. Therefore,the V_(NGI) control logic of FIG. 4a, and corresponding signalsequencing logic of FIG. 4c allows for additional tailoring of thecharacteristics of the pre-erase/incremented erase operation for anyparticular environment or application.

FIG. 5 illustrates a first embodiment of a single-shot pulse generatorcircuit 500 that can be used with the present invention. An input signalIN is provided to a first series of conventional inverters 502, 504 in afirst signal path, while a second series of conventional inverters 506,508 and 510 along with capacitive elements 512 and 514 are used in asecond, delayed signal path. The OUT signal of NAND gate will track theIN signal in the manner illustrated in this figure, and the delayelements can be easily manufactured using conventional processing toreasonable accuracies needed to help control the pre-erase/incrementederase conditioning signal behavior. In FIG. 6, a second embodiment of apulse generator circuit 600 is depicted. In this variation, input signalIN is provided to a first series of conventional inverters 602, 604 in afirst signal path, while a second path is comprised of a variable delayinverter stage formed of PMOS FET 606, NMOS FET 608, and resistance R.This stage is also followed by conventional inverters 614, 616, and acapacitive element 612. The OUT signal of NAND gate will track the INsignal in the manner illustrated in this figure, and the delay will be afunction of the RC constant of the second signal path. The benefit ofthis embodiment lies in the fact that the delay characteristics can becustomized on a chip by chip basis during the manufacturing process byconventional laser trimming operations performed on a variable resistor.

FIGS. 7A to 7C illustrate in rough graphical form the effect of thepresent invention on a typical flash memory cell array. It will beunderstood by those skilled in the art that these graphs are merelyapproximations and are not intended to reflect the actual behavior anyparticular array. Nevertheless, applicant believes that theseillustrations help clarify the operational features, effects andbenefits of the present inventions.

First, in FIG. 7A, voltage threshold distributions are plotted for aflash memory population in an erased state (lower threshold voltage) andprogrammed state (higher threshold voltage). It should be noted firstthat these terms (erased, programmed) are merely descriptive labels forthe charge condition of the floating gates, and it is entirely possiblethat in some applications these labels will be reversed for the samecharge condition. In other words, the present invention can be practicedeven in environments where the program/erase logic is opposite to thatshown herein, since the operational features and benefits can still beemployed effectively wherever it is desirable to equalize cell dischargerates. In any event, flash memory cells in a programmed state ideallyhave a voltage threshold V_(t)=V_(p), where V_(P) is a target programmedvoltage. In reality, however, the programmed threshold voltagedistribution looks substantially like that shown in FIG. 7A, where thereis an approximately normal distribution (solid line) centered aroundvoltage level V_(p). So long as the cells have threshold values betweenV_(pmin) and V_(pmax), nonetheless, they will still be adequatelydiscriminated by the sense amplifiers of the flash device as being in aprogrammed state. Similarly, flash memory cells in an erased stateideally have a voltage threshold V_(t)=V_(e), where V_(e) is a targeterased voltage. Again, in real world devices, chip designers are contentto operate with the erased threshold voltage distribution lookssubstantially like that shown in the FIG. 7A, where there is anapproximately normal distribution (solid line) centered around voltagelevel V_(e). As above, so long as the cells have threshold valuesbetween V_(emin) and V_(emax), such cells will still be adequatelydiscriminated by the sense amplifiers of the flash device as being in anerased state.

A serious problem, however, lies in the fact that the flash cells in thearray do not behave consistently or uniformly during an erase operation.A slow bit, identified in FIG. 7a, will change threshold voltages at arate far slower than a fast bit identified in FIG. 7a. These fast bitstend to become over-erased, and the slow bits tend to be under-erased,as evidenced by the dotted line distribution shown in FIG. 7a. For avariety of well-known reasons, the effect is not symmetric, however, sothat the slight under-erasure of the slow bits does not usually presenta significant problem. In other words, they are still discernible as“erased,” so long as their threshold voltage is less than V_(temax), andthey do not contribute to any deleterious leakage currents as is thecase with over-erased bits. In contrast, the fast bits can becomeover-erased, and therefore the population of cells within the shadedarea of FIG. 7a must normally be dealt with using the correctiveprocedures described above. Still, this after-the-fact correction isundesirable and reduces device life span.

The benefits of the present invention on the cell threshold distributioncan be seen in FIG. 7b. This shows the programmed state of the array,after the pre-erase operations have taken place. The top end of thevoltage thresholds are curtailed, significantly, so that thedistribution is tighter. The same fast bit has now had a thresholdvoltage alteration from V_(f) to V_(f)′, where V_(f)′<V_(f); this meansthat this same bit now acts more like an average cell, and is far lesslikely to be over-erased during the subsequent erase operation (shownapproximately as the dotted line). The slow bits and average programmedbits, in contrast, are hardly affected, and therefore the overall eraseoperation is improved significantly from a variety of perspectives.

In FIG. 7C, a graphical comparison of cell discharge rates for fast,normal, and slow bits is given. Because of the intense electrical fieldsin fast bits, FN tunneling occurs much faster, so they discharge muchfaster compared to normal and slow bits as well. At the end of an eraseoperation (time T_(e)) such bits are well past being erased (V_(e)), andin fact are over-erased (below V_(emin)). By pushing the thresholdvoltage Vf to a more normal threshold voltage V_(N) at the start of theerase operation, this bit now is more likely to behave like such normalbit, and therefore result in a correctly erased bit at the end of theerase operation.

FIG. 8 is an embodiment of a well bias voltage circuit 800 that is usedin conjunction with the above pre-erase/incremented erase operations.This circuit is generally responsible for ramping the voltage state ofthe N-well and P-well regions of the array during these operations tofacilitate the reduction of the field intensity across the tunnel oxidesof the fast bits. To accomplish this, a voltage signal V_(Gwell) isgenerated and applied by well bias voltage circuit at the same time theaforementioned V_(GER) conditioning signal is applied to the controlgates of the cells in the array. The characteristics of the well biassignal can be configured to the particular array in the followingmanner: first, the overall capacitance of the wells C_(L) is generallyknown; to achieve a particular desired voltage change ΔV in the well ina particular time ΔT, as shown in FIG. 8A, a constant current source Ican be used, where I=C_(L)*ΔV/ΔT. Thus, given these parameters, a wellbias voltage circuit 800 can be constructed in accordance with thecircuit elements illustrated in FIG. 8. These include a select gate 805,which is triggered on either a sector erase (SECERS) or chip erase(CHPERS) signal. A bias level circuit 808 for generating a bias signalI′ includes: a driver 810 coupled to a n type switch 811 to a firstvoltage terminal (Vss); the latter is in series with an inverter stagecomprised of p type switch 812 and n type switch 813, and a p-typeswitch 814 connected through to a second voltage terminal (Vdd). A finaln type switch 815, connected to Vss, and a capacitor 816 complete thisportion of the well bias circuit. A series of p-channel FETs 820, 821,822, 823 and 824 receive bias signal I′ and have a first terminalcoupled to a high voltage positive pump signal VPPI (generated inconventional fashion from a charge pump) from a source. Output VGwell istaken at a second terminal of these p-channel FETs. In a preferredembodiment, the number and selection of FETs 820-824 that contribute tothis signal is configured during manufacture of the device based onmeasured/desired characteristics of such signal. Thus, in the exampleshown in FIG. 8, only two of the p-channel FETs (820 and 821) actuallycontribute to the V_(Gwell) signal in this embodiment, but it isunderstood that any or all of such FETs (or a different number of FETsbeyond that shown in this figure) can be configured by suitableprocessing selections (laser cutting for example) based on theparticular requirements needed for signal V_(Gwell). This circuitoperates generally as follows: when an erase operation is selected, ntype switch 815 is turned on, while p type switch 812 and n type switchare both turned off. This pulls down the gate terminal (node) of p typeFETs 820 and 821 to Vss (ground), turning them on. This event in turn,couples the output terminals of such FETs to rise based on the terminalvoltage V_(PPI), and causing V_(Gwell) to begin rising at a relativelyconstant rate based on the RC time constant resulting from capacitor 816and the impedance of the two FETs. After a time T, therefore, V_(Gwell)(and therefore the N and P wells) will attain a final value Vf, which,again, can be tailored for the specific array. This value of Vf, again,can be empirically determined based on testing and measurements of thearray so that it contributes to optimize the effects of theaforementioned conditioning signals V_(GER). In a preferred embodiment,applicants have discovered that a value of approximately - - - voltsappears to work well as a well bias voltage signal level. As can be seenin FIG. 8, when there is no erase operation taking place, n type switch815 is turned off, but n type switch 811 and p type switch 814 areturned on. This causes the gate terminal of p type FETs 820 and 821 tofloat, which means that such devices will not be turned on, and theV_(Gwell) signal will float, and thus also be essentially isolated fromthe rest of the circuit 800. Other variations and implementation forthis well bias voltage circuit will be apparent to those in the art fromthe above teachings, and the present invention is certainly not limitedin any respect by any such particular adaptations. It will beimmediately understood by those skilled in the art that a flash memorycell array integrated circuit can be manufactured using conventionalprocessing means to include the teachings of the present invention. Sucharticle of manufacture could include the inventive methods embodied asreplacements for embedded algorithms in known control circuits, as wellas usual combinations of flash memory cell arrays and conventionalsupporting peripheral circuitry (power supplies, address decoders, I/Odata buffers, sense amplifiers, reference arrays, counters, timers,etc.). Such processing means and peripheral circuitry can be implementedusing any of a number of structures and methods well-known in the art,and are therefore not described here in substantial detail.

It will also be appreciated by those skilled in the art that the abovediscussion is directed to a preferred embodiment of the presentinvention, and that the present teachings can be used in a number ofdifferent environments, applications, etc. For example, it is possiblethat other flash memory cell operations (other than erasures) maybenefit from a conditioning operation that adjusts the electrical fieldacross the tunnel oxide, or the amount of electrical charge on thefloating gates. Similarly, while the present detailed descriptionillustrates a mechanism for slowing down fast bits by removing charge,it is of course possible to also speed up slow bits by adding charge,and this feature, too, may be attractive for some applications.Accordingly, it is intended that the all such alterations andmodifications be included within the scope and spirit of the inventionas defined by the following claims.

What is claimed is:
 1. A method of equalizing erase rates of flash cellsin a memory array, the flash cells each having a floating gate with anfloating gate charge value, and which floating gate charge value isintended to be set to a target initial charge value prior to an eraseoperation, the method comprising: (a) generating a flash cellconditioning signal having a predetermined magnitude and duration; (b)adjusting the floating gate charge value of each of such flash cellsusing said conditioning signal, the floating gate charge value for anysuch flash cell being adjusted by said conditioning signal by a chargeamount proportional to the floating gate charge value; wherein eraserates of flash cells with floating gate charge values substantially inexcess of said target initial charge value are substantially affected bysaid flash cell conditioning signal, while erase rates of flash cellswith floating gate charge values substantially below said target initialcharge values are substantially unaffected by said flash cellconditioning signal; and further wherein steps (a) and (b) are repeateduntil the flash cell floating gate charge values substantially convergeto the target initial charge value so that all of the flash cells canthen be erased at an approximately uniform erase rate by a subsequenterase operation.
 2. The method of claim 1, wherein said conditioningsignal is altered to remove a greater quantity of charge for each timesteps (a) and (b) are repeated.
 3. The method of claim 1, whereincharacteristics of said conditioning signal are adjusted during amanufacturing operation so that they are tailored for such memory array.4. The method of claim 1, wherein said predetermined magnitude of saidflash cell conditioning signal is controlled by a state of a pluralityof logic signals.
 5. The method of claim 1, wherein said flash cellconditioning signal is based on a negative voltage reference signalderived from a negative charge pump.
 6. The method of claim 1, wherein atime sequencing of said flash cell conditioning signal is controlled bya sequence control logic signal.
 7. The method of claim 4, wherein saidpredetermined magnitude of said flash cell conditioning signal isincreased by a fixed amount during each time steps (a) and (b) arerepeated.
 8. The method of claim 1, wherein said flash cell conditioningsignal is applied a variable number of pulses during step (b), saidvariable number of pulses being adjusted according to manufacturingcharacteristics of the memory array.
 9. A circuit for equalizing eraserates of flash cells in a memory array, the flash cells each having afloating gate with an floating gate charge value, and which floatinggate charge value is intended to be set to a target initial charge valueprior to an erase operation, the circuit comprising: (a) a flash cellcontroller circuit configured to generate a set of control signals forcontrolling a magnitude, duration and sequence associated with a flashcell conditioning signal; (b) a flash cell conditioning signal generatorfor generating said flash cell conditioning signal in response to saidset of control signals; and wherein said flash cell conditioning signalis used for adjusting the floating gate charge value of each of suchflash cells and operates to adjust the floating gate charge value forany such flash cell by a charge amount proportional to an originalstarting floating gate charge value; and further wherein erase rates offlash cells with floating gate charge values substantially in excess ofsaid target initial charge value are substantially affected by saidflash cell conditioning signal, while erase rates of flash cells withfloating gate charge values substantially below said target initialcharge values are substantially unaffected by said flash cellconditioning signal.
 10. The circuit of claim 9, wherein said set ofcontrol signals is varied during a time sequence so that a series offlash cell conditioning signals are applied during said time sequence tosaid flash cells, and such that the flash cell floating gate chargevalues substantially converge to the target initial charge value so thatall of the flash cells can then be erased at an approximately uniformerase rate by a subsequent erase operation.
 11. The circuit of claim 9,wherein a probability of any cell becoming over-erased from the fullstrength erase signal is reduced.
 12. The circuit of claim 9, whereinsaid magnitude of said flash cell conditioning signal is controlled by astate of a plurality of logic signals.
 13. The circuit of claim 9,wherein said flash cell conditioning signal is based on a negativevoltage reference signal derived from a negative charge pump.
 14. Thecircuit of claim 9, wherein said sequence associated with said flashcell conditioning signal is controlled by a sequence control logicsignal.
 15. The circuit of claim 9, wherein said magnitude of said flashcell conditioning signal is increased by a fixed amount during eachsequence when said flash cell conditioning signal is generated.
 16. Thecircuit of claim 9, wherein said flash cell conditioning signal isapplied a variable number of pulses during each said sequence, saidvariable number of pulses being adjusted according to manufacturingcharacteristics of the memory array.